Field of the Invention
The present invention relates to an optical filter and a device using an optical filter.
Discussion of the Background
In solid-state image pickup devices, such as video cameras, digital still cameras, cellular phones having camera function and smart phones, a CCD or CMOS image sensor that is a solid-state image sensor of color image is used. In such a solid-state image sensor, silicon photo diode having sensitivity to near-infrared rays that cannot be perceived by human eye is used in its light-receiving section. For such a solid-state image sensor, it is necessary to make correction of visibility so that a natural color might be obtained when an image is seen with human eye, and an optical filter (e.g., near-infrared cut filter) to selectively transmit or cut rays of specific wavelength region is frequently used.
As such near-infrared cut filters, those produced by various processes have been used in the past. For example, in JP-A-H6-200113, a near-infrared cut filter using a substrate formed of a transparent resin and containing a near-infrared absorbing agent in the transparent resin is described, and in Japanese Patent No. 5036229, a near-infrared cut filter using a glass substrate containing a copper ion is described.
On the other hand, an attempt to impart a sensing function utilizing near-infrared rays, such as motion capture or distance recognition (space recognition), to a camera module has been made in recent years. In such a use, it becomes necessary to selectively transmit visible rays and a part of near-infrared rays, and therefore, near-infrared cut filters that indiscriminately shield near-infrared rays, such as conventional ones, cannot be used.
As optical filters that selectively transmit visible rays and a part of near-infrared rays, filters having a dielectric multilayer film formed on a glass substrate have been sold by Toa Optical Technologies, Ltd., Ceratec Japan Co., Ltd., etc.
With regard to optical filters that are on the market as the aforesaid optical filters selectively transmitting visible rays and a part of near-infrared rays, their spectral properties greatly vary when rays are obliquely incident on the filters. Especially in the case where the dependence of an optical filter on the incident angle is large on the long wavelength side of a visible light passband and on the short wavelength side of a near-infrared ray selective passband, a signal noise ratio (S/N ratio) of near-infrared rays used for the sensing function is deteriorated when the rays are obliquely incident on the filter, and besides, there is a problem that evil influence is exerted on camera image quality or color reproducibility at the image end part.